We have been using Resilio Sync 2 for years in our family and recently upgraded to Sync 3. We only have thousands, not millions of files in sync, but for those I like about it that in my experience Resilio Sync “just works”… regardless of firewalls, getting a new computer, accessing or storing files on mobile etc.
I also run the Linux version on an always-on server for better syncing and you can leave it running for weeks on end without issues.
Archive link: http://archive.today/ythmV